Epitaxial wafer and manufacturing method thereof as well as light-emitting diode with enhanced luminance

ABSTRACT

An epitaxial semiconductor crystal plate or wafer capable of attaining increased reliability with enhanced luminance, a manufacturing method thereof as well as a light-emitting diode (LED). It has been found that epitaxial wafers with enhanced illuminance and increased yield of manufacture can be fabricated by specifically arranging a double-heterostructure epitaxial wafer such that the interface between its p-type clad layer 2 and p-type GaAlAs active layer 3 and that between an n-type GaAlAs clad layer 4 and p-type GaAlAs active layer 3 measure 1×10 17  cm -3  or less in oxygen concentration. Also, in order to cause the oxygen concentration near the p-type GaAlAs active layer 3 in layers of the epitaxial wafer to be less than or equal to 1×10 17  cm -3 , it may be preferable that a nondoped GaAs polycrystal for use as a preselected original material in liquid-phase epitaxial growth be less than or equal to 1×10 16  cm -3  or thereabout.

BACKGROUND OF THE INVENTION

The present invention generally relates to optical semiconductormicroelectronics, and more particularly to epitaxial crystal multilayerstructures and a manufacturing method thereof. The invention alsorelates to semiconductor light-emitting devices with enhanced lightemission output intensity.

In recent years, red light-emitting diodes (LEDs) have become morewidely used in the manufacture of electronic equipment. Several types ofLEDs have been developed to date. A double-heterostructure LED is oneexample. A rear-surface reflection LED is another example. These LEDscome with enhanced luminance, and hence offer extended usability andapplicability, such as for use with indoor lamps and display devices, aswell as outdoor display devices.

However, in spite of the foregoing advances in this technology, it isalso true that there are continuous demands for a further increase inluminance and a further decrease in manufacturing cost. To satisfy suchdemands, great efforts have been made to study several subjectsinvolving the film thickness of the epitaxial layer, the carrier densityor concentration, the crystal growth temperature settings, and otherfactors. Unfortunately, no efforts have been reported as beingsuccessful in demonstrating significant advantages leading toaccomplishment of further improvements luminance at least at present.

For these reasons, it still remains difficult to meet luminancespecifications as demanded, which in turn makes it difficult to providean improvement in the manufacturing yield, while reducing the coststherefor.

In view of the foregoing, it may be appreciated by those skilled in thesemiconductor art that there remains an unmet need for an epitaxialwafer capable of offering highly enhanced luminance with increasedstability and reliability, along with an improved manufacturing method,as well as a light-emitting diode using the same.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved optical semiconductor structure which avoids the problemsencountered with the prior art.

It is another object of the invention to provide an epitaxial waferstructure which is capable of increased performance and reliability.

It is yet another object of the invention to provide an improved methodof forming an epitaxial multilayer lamination wafer which is capable ofincreased light emission output and reliability while reducing thecomplexity of manufacture and the costs therefor.

It is still another object of the invention to provide an improvedlight-emitting device which is capable of increased light emissionoutput and reliability while reducing the complexity of manufacture andthe costs therefor.

It is a further object of the invention to provide ahigh-performance/high-reliability light-emitting multilayer epitaxialsemiconductor device structure which is capable of maximizing the outputlight luminance characteristics and light emission stability, whilereducing the complexity and costs.

To attain the foregoing objects, in accordance with one aspect of thepresent invention, a double-heterostructure epitaxial crystal plate orwafer is provided which has on its p-type compound semiconductorsubstrate a multilayer lamination including a p-type compoundsemiconductor clad layer, a p-type compound semiconductor active layerand a n-type compound semiconductor clad layer which are sequentiallyformed by liquid-phase epitaxial growth techniques. Each interfacebetween the p-type compound semiconductor clad layer and p-type compoundsemiconductor active layer and between the p-type compound semiconductoractive layer and n-type compound semiconductor clad layer measures1×10¹⁷ atoms per cubic centimeter (cm⁻³) or less in oxygenconcentration.

With such specific numerical control of the oxygen concentration near oraround the active layer, it becomes possible to adjust the lightemission output intensity, which in turn ensures that the light outputintensity decreases with an increase in oxygen concentration, whereas itincreases with a decrease in oxygen concentration. Accordingly, lettingthe oxygen concentration be set at 1×10¹⁷ cm⁻³ may enable the lightemission luminance to increase by 30%, or more or less, as compared withthe prior art. Preferably, the compound semiconductor materials for usein forming the multilayer wafer structure may be gallium arsenide(GaAs), gallium phosphide (GaP), gallium indium phosphide (GaInP), orequivalents thereto. Note here that the principles of the invention mayalso be applicable to epitaxial wafers with a rear-surface reflectionstructure, which eliminates use of the p-type compound semiconductorsubstrate from the multilayer structure mentioned supra.

In accordance with a further aspect of the invention, the foregoingdouble-heterostructure epitaxial wafer may be fabricated or manufacturedby a specific method as follows. While employing a non-doped compoundsemiconductor polycrystalline having an oxygen density or concentrationless than or equal to 1×10¹⁶ cm⁻³, sequentially grow by liquid-phaseepitaxial techniques a p-type compound semiconductor clad layer, ap-type compound semiconductor active layer and an n-type compoundsemiconductor clad layer on a p-type compound semiconductor substrate inthis order. During fabrication, each interface between the p-typecompound semiconductor clad layer and p-type compound semiconductoractive layer and the interface between the p-type compound semiconductoractive layer and n-type compound semiconductor clad layer arespecifically set to be less than or equal to 1×10¹⁷ cm⁻³ in oxygenconcentration. With such a fabrication technique, it is possible tomanufacture a high-performance epitaxial wafer having a maximized lightemission luminance with increased production yield and reduced costs.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing in cross sect a double-heterostructureepitaxial wafer in accordance with one embodiment of the invention.

FIG. 2 is a graphical representation showing a distribution of oxygenconcentration near an active layer of one prior artdouble-heterostructure epitaxial wafer.

FIG. 3 is a graph showing a relation of light emission output versus theoxygen concentration of the interface between adjacent clad and activelayers of a double-heterostructure epitaxial wafer embodying theinvention.

FIG. 4 is a graph showing a distribution of oxygen concentration near anactive layer of the double-heterostructure epitaxial wafer embodying theinvention.

FIG. 5 is a diagram showing, in cross-section, a rear-surface reflectionepitaxial wafer in accordance with a further embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Some preferred embodiments of the present invention will now bedescribed with reference to the accompanying figures of the drawing. Inthe illustrative embodiments, a double-heterostructure epitaxial waferand an epitaxial wafer with rear-surface reflection structure arespecifically configured in structure so as to increase the luminance byapproximately 30% as compared with the prior art thus attaining highlyenhanced luminance characteristics.

Referring now to FIG. 1, a double-heterostructure epitaxial wafer inaccordance with one preferred embodiment of the invention is illustratedin cross section. As shown in FIG. 1, the double-heterostructureepitaxial wafer includes a p-type GaAs substrate 1 on which a p-typeGaAlAs clad layer 2 is formed. Formed on this GaAlAs clad layer 2 is ap-type GaAlAs active layer 3 having a surface on which an n-type GaAlAsclad layer 4 is formed.

FIG. 2, is a graph showing the distribution of oxygen concentration nearthe p-type GaAlAs active layer 3 of the aforesaid double-heterostructureepitaxial wafer. The measurements of the distribution shown herein weredone in such a way that the secondary ion mass spectroscopy (SIMS)method was employed to measure the oxygen concentration along the depthof epitaxial layers. This SIMS method is such that each specimen orsample is doped and hit with primary ions having an energy ranging fromseveral kilovolts (kV) to 20 kV at minute points or "micropoints"thereof causing a surface material to be sputter-ionized, thereby topermit analysis using mass analyzer equipment.

The measurement results reveal the fact that a peak of oxygenconcentration measuring 1×10¹⁸ atoms per cubic centimeter (cm⁻³) ortherearound exists at the interface region between the p-type GaAlAsclad layer 2 and p-type GaAlAs active layer 3 while a similar peak of2×10¹⁷ cm⁻³ or near is present at the interface between the n-typeGaAlAs clad layer 4 and the GaAlAs active layer 3.

It has also been found that in this double-heterostructure epitaxialwafer, it is possible, by specifically setting at 1×10¹⁷ cm⁻³ or lessthe oxygen concentration at or near the interface between the p-typeGaAlAs clad layer 2 and p-type GaAlAs active layer 3, as well as theoxygen concentration at or near the interface between n-type GaAlAs cladlayer 4 and p-type GaAlAs active layer 4, to attain an epitaxial waferwith enhanced luminance and increased manufacturing yield.

It has further been found that in order to let the oxygen concentrationinside the epitaxial wafer layers be less than or equal to 1×10¹⁷ cm⁻³,it may be recommendable to employ, as a preselected original materialfor use in liquid-phase epitaxial growth, a nondoped GaAspolycrystalline material which measures 1×10¹⁶ cm⁻³ or less in oxygenconcentration. It has moreover been found that an increase in flow rateof ambient gas--here, hydrogen gas--is preferable.

Actually reduced embodiments incorporating the principles of theinvention will be described in detail hereinafter. First of all, somepreferred embodiment will be explained in a case where in order tofabricate a 660-nm wavelength double-heterostructure LED, epitaxialwafers are prepared by changing among various values the oxygenconcentration at the interface between the p-type GaAlAs clad layer 2and p-type GaAlAs active layer 3.

Prepare a graphite board. Set on this board a p-type GaAs substratealong with several original materials constituting respective LEDlayers. Then, put the resulting broad structure into a liquid-phaseepitaxial furnace. By way of example, these layer materials may firstinclude a p-type clad layer material for setting Ga, GaAs polycrystal,Al and Zn. Secondly, the materials may include a p-type active layermaterial for setting Ga, GaAs polycrystal, Al and Zn. Furthermore, theymay include an n-type clad layer material for setting Ga, GaAspolycrystal, Al and Te.

Then, heat structure in the atmosphere of hydrogen gas up to apredetermined temperature of 950° C., for example. Next, control thefurnace temperature to drop down at a predefined falling rate of 1.0° C.per minute thereby sequentially forming on the p-type GaAs substrate thefollowing three layers: a p-type GaAlAs clad layer 2 grown to athickness of 25 micrometers (sum); an overlying p-type GaAlAs activelayer 3 grown to a thickness of 1 Mm; and a further overlying n-typeGaAlAs clad layer 4 grown to a thickness of 40 μm.

With the epitaxial growth method stated above, seven pieces (seven rots)of epitaxial wafers were prepared which were 1×10¹⁸ cm⁻³, 8×10¹⁷ cm⁻³,6×10¹⁷ cm⁻³, 4×10¹⁷ cm⁻³, 2×10¹⁷ cm⁻³, 1×10¹⁷ cm⁻³ and 8×10¹⁶ cm⁻³ inoxygen concentration at the interface between the p-type GaAlAs cladlayer 2 and p-type GaAlAs active layer 3.

The oxygen concentration was adjusted in value by use of GaAspolycrystals having different flow rates of hydrogen gas flowing insideof the furnace and being in oxygen concentration. Thereafter,appropriate electrodes are formed on a respective one of the resultantcrystal-grown epitaxial wafers. Finally, resin-molding is performedthereby to complete the LEDs.

FIG. 3 is a characteristic diagram demonstrating the relation of lightemission output of the LEDs thus manufactured versus oxygenconcentration. This was measured with its forward current If set at 20milliamperes (mA). In the graph of FIG. 3, the light emission outputincreases with a decrease in oxygen concentration at the interfacebetween the p-type GaAlAs clad layer 2 and p-type GaAlAs active layer 3;and when the concentration is at 1×10¹⁷ cm⁻³, the light emission outputis held substantially constant.

When compared with an LED with oxygen concentration of 1×10¹⁸ cm⁻³, thisembodiment offered a 30%-increase in light emission luminance. To rendersuch interface oxygen concentration less than or equal to 1×10¹⁷ cm⁻³,use of a specific GaAs polycrystal is required in which theconcentration of oxygen content is not greater than 1×10¹⁶ cm⁻³.

See FIG. 4. This graph demonstrates an SIMS analysis result with regardto an LED as manufactured to be less than or equal to 1×10¹⁷ cm⁻³ inoxygen concentration at the interface thereof. In the graph of FIG. 4,the oxygen concentration at both interfaces--i.e. the interface betweenp-type GaAlAs clad layer 2 and p-type GaAlAs active layer 3, and thatbetween n-type GaAlAs clad layer 4 and p-type GaAlAs active layer3--were 1×10¹⁷ cm⁻³ or less.

With such an arrangement, it becomes possible to successfully controlthe light emission output by adjusting the oxygen concentration near oraround the p-type GaAlAs active layer 3. This may enable highlyefficient manufacture of LED devices which are enhanced by 30% or morein light emission output intensity, as compared with the prior art LEDs,while at the same time increasing the yield of production and reducingthe manufacturing costs.

Turning now to FIG. 5, a rear-surface reflection epitaxial wafer inaccordance with a further embodiment of the invention is shown. Thewafer is similar in structure to that shown in FIG. 1 with the p-typeGaAs substrate 1 being removed away. With such an LED structure also,the resultant luminance was capable of being increased by approximately30%.

In the foregoing illustrative embodiments, specific examples arepresented which employ certain GaAs polycrystalline materials with lowconcentration of oxygen content. However, it may be appreciated by thoseskilled in the semiconductor art that some other causes may beconsidered for letting the prescribed interface oxygen concentrationincrease. For instance, a decrease in purity of ambient gas used is onepossible cause. An insufficiency of refining or purification of graphitetools is another possible cause. An accidental oxidization of originalmaterials and/or dopants is a further cause. Avoiding these causes mayenable an improvement in interface oxygen concentration.

Furthermore, while the illustrative embodiments are describedhereinbefore based on specific examples employing GaAlAs as the compoundsemiconductor thereof, it may be appreciated by those skilled in the artthat the compound semiconductor may alternatively comprises otherequivalent semiconductive materials, including, but not limited to, GaAsand GaP, GaInP.

As has been described, the present invention provides a specificepitaxial wafer with double-heterostructure or with a rear-surfacereflection structure, wherein the interface between the p-type compoundsemiconductor clad layer and the p-type compound semiconductor activelayer and the interface between the p-type compound semiconductor activelayer and the n-type compound semiconductor clad layer are each lessthan or equal to 1×10¹⁷ cm⁻³ in oxygen concentration, thereby enablingresultant light emission luminance to increase by 30% or thereabouts.

While the invention has been described with reference to specificembodiments, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A double-heterostructure epitaxial semiconductordevice comprising:a compound semiconductor substrate of a predefinedtype of conductivity; a multilayer lamination on said substrate, saidlamination including a first compound semiconductor clad layer of thepredefined conductivity type, a compound semiconductor active layer ofsaid predefined conductivity type, and a second compound semiconductorclad layer of a conductivity opposite said predefined conductivity type;the first clad layer and the active layer having a first interface asdefined therebetween; said active layer and the second clad layer havinga second interface therebetween; and each of the first and secondinterfaces being less than or equal to 1×10¹⁷ atoms per cubic centimeter(cm⁻³) in oxygen concentration.
 2. The device of claim 1 wherein saidsubstrate and the compound semiconductor layers include one selectedfrom the group consisting of gallium arsenide (GaAs), gallium phosphide(GaP), and gallium indium phosphide (GaInP).
 3. The device of claim 2wherein the first conductivity type is p-type whereas the secondconductivity type is n-type.
 4. An epitaxial semiconductor device withrear-surface reflection structure, said device comprising:a firstcompound semiconductor clad layer of a first type of conductivity; amultilayer lamination on the first clad layer, said lamination includinga compound semiconductor active layer of the first conductivity type anda second compound semiconductor clad layer of a second type ofconductivity; said first clad layer and said active layer having a firstinterface as defined therebetween; said active layer and the second cladlayer having a second interface therebetween; and each of the first andsecond interfaces being less than or equal to 1×10¹⁷ cm⁻³ in oxygenconcentration.
 5. The device of claim 4 wherein the compoundsemiconductor layers include one selected from the group consisting ofGaAlAs, GaP and GaInP.
 6. The device of claim 5 wherein the firstconductivity type is p-type whereas the second conductivity type isn-type.
 7. A light-emitting diode making use of an epitaxial wafer of arear-surface reflection type, said wafer comprising:a first compoundsemiconductor clad layer of a first conductivity type; a multilayerlamination on the first clad layer, said lamination including a compoundsemiconductor active layer of the first conductivity type and a secondcompound semiconductor clad layer of a second conductivity type; saidfirst clad layer and said active layer having a first interface astherebetween; said active layer and the second clad layer having asecond interface therebetween; each of the first and second interfacesbeing less than or equal to 1×10¹⁷ cm⁻³ in oxygen concentration; ananode electrode on said first clad layer; and a cathode electrode onsaid second clad layer.
 8. The diode of claim 7 wherein the firstconductivity type is p-type whereas the second conductivity type isn-type.